In the past, a typical application for copiers or scan-to-print image processing systems was to reproduce an input image as accurately as possible, i.e., render a copy. Thus, copies have been rendered as accurately as possible, flaws and all. However, as customers become more knowledgeable in their document reproduction requirements, they recognize that an exact copy is often not what they want. Instead, they would rather obtain the best possible document output. Until recently, image quality from the output of a copier or a scan-to-print system was directly related to the input document quality. One very common set of input documents includes photographs. Unfortunately, photography is an inexact science, particularly among amateurs, and original photographs are often poor. Alternately, technology, age or image degradation variations result in pictures having an unsatisfactory and undesirable appearance. What is desired then, is a copy giving the best possible picture, and not a copy of the original.
Three possible choices are presented in the area of image enhancement. In the first case, we can do nothing. Such a system is a stable system, in that it does no harm to an image. This is a common approach taken to reproduction. However, the output documents of such a system are sometimes not satisfactory to the ultimate customer.
In a second case of image enhancement, the image can always be processed. It turns out than an improvement can usually be made to an image if certain assumptions are made that are accurate for most cases. This model fails in multi-generation copies, or where the original image was in fact, correct.
Accordingly, we arrive at our third case of image enhancement, a process of automated image enhancement which operates to vary images which are not perceived as good images, but does not operate on images which do not need to be improved.
One improvement that can be made to an image is correction of color shifts. When photographic color prints (a significant image source for electronic images) are made from negative or positive color images, the overall colors of the print frequently do not correspond to those desired. This may arise from a number of causes, such as change in sensitivity of the film due to aging, use of incorrect lighting, error in print processing and the like. If an exact print is made from such pictures based on the sensitivity which the film should have had, i.e., assuming that the positive or negative was correct, then the printer, particularly if a reflection prints (as opposed to a transparency) will display the errors to a greater extent than the transparency because such printers are nearly always viewed under conditions in which there are comparison objects. Of course, electronic images can only reproduce what is recorded, but given the possibility of pixel by pixel color editing.
U.S. Pat. No. 2,571,697 to Evans teaches that in photographic processes, an overall color shift can be made to the image without knowledge of the original colors. Starting with the assumption that if light passes though the printing apparatus onto a printing material without a color image or other obstruction in the light beam, the printing light should produce a neutral gray (approximately). For a color shifted image, if light which will reproduce substantially as gray on the printing material is permitted to pass though the transparency so that a uniform amount of the light strikes the transparency at all points, the light passing through the transparency usually will not print as neutral gray, but will deviate from gray by an appreciable amount. The light which passes through the transparency is collected or integrated and each component primary color forming the light is measured by the use of a photoelectric cell. By comparing the amount of these colors received after passing through the transparency with the amount of these colors in light from the light source, a correction factor can be determined, and an adjusted light source can be provided, by inserting a filter in the light path.
It will no doubt be appreciated that the photographic correction process described in U.S. Pat. No. 2,571,697 to Evans is limited to changes across the entire image, limited to filters available; limited to corrections to each component color, and limited to uniform analog changes over the entire input intensity range.